Process for producing CuCr contact pieces for vacuum switches as well as an appropriate contact piece

ABSTRACT

Purely powder-metallurgical processes or sinter-impregnation processes are often used to manufacture CuCr contact materials. Here the aim is to obtain the lowest possible residual porosity, which should be &lt;1%. According to the invention, a powder moulding of the components is densified in two stages; the first stage is a sintering process with a densification of the sintered body to a closed porosity, and the second stage is a hot-isostatic pressing operation (HIP), in which the unencased workpieces are taken to a final density amounting to a space occupation of at least 99%. Thus, an economical method of manufacturing high grade material is obtained. It is possible to produce multi-layer contacts or self-adhesive bonds between the sintered body and a solid substrate, e.g. a copper contact bolt.

BACKGROUND OF THE INVENTION

The present invention relates to a process for producing a contact pieceusing copper and chromium for applications in vacuum-switch tubes, inwhich a powder blank is compacted from the starting components rightdown to a residual porosity of <1%, as well as to a contact pieceproduced in such a way.

Composite materials consisting of a conductive component and at leastone high-melting component and, if necessary, also containing additivesthat lower welding force or reduce chopping current have proven theirworth as contact materials for vacuum-switch tubes. The widely used CuCrmaterials are a typical example of this.

Since a high-melting component such as chromium only has a lowsolubility in the electrically conductive main component such as copper,powder-metallurgical processes are highly considered for manufacturingCuCr contact materials.

A process that is often applied to produce such contact materials is thesintering of a Cr skeleton and the subsequent infiltration of thesintered skeleton with Cu. This is described, for example, in the Germanpatent applications DE-A-25 21 504 or the DE-B-25 36 153. The result inthis case is that qualitatively high-grade materials with good switchingproperties can be obtained.

However, this process is susceptible to defects and requiresconsiderable expenditure for quality assurance. Since a liquid phase isused, the blanks that are formed are clearly oversized and requiremachining to obtain the final form. Moreover, the requirement for aself-supporting skeleton means that the concentration range available tothe high-melting component is restricted.

The last mentioned disadvantages can be avoided by using anotherwidespread process, in which a powder mixture of the components ispressed or sintered and then still cold or hot afterpressed. This isdescribed, for example, in the applications DE-A-29 14 186, the DE-A-3406 535 and the EP-A-0 184 854. In this process, the concentration of thecomponents can be selected within broad limits and the contour of theblanks can be set close to the final form, since extended cavitysystems, such as those that can develop in poor impregnation materials,do not occur in the material. However, such materials have a residualporosity, usually between 4% and 8%, which has a disadvantageous effecton their application as a contact material for contact pieces ofvacuum-switch tubes. The reason for this is that with increasingporosity, the danger of later breakdowns escalates and thebreaking-capacity limit diminishes, whereby the welding tendency goesup.

From the EP-A-0 184 854, it is already known to compact the powderbodies not by means of solid-phase sintering, but rather by hot-pressingthem. As a result, materials are produced which have a negligibly lowresidual porosity and which avoid the above-mentioned disadvantages.However, this manufacturing method must take place under a vacuum or inhighly purified protective gas and is therefore cost intensive and thusrelatively uneconomical.

DE-A-37 29 033 discloses another manufacturing method for CuCr contactmaterials, in which a solid-phase sintering step is combined with ahot-isostatic liquid-phase compressing step (HIP). One starts out fromsintered bodies that have a relatively low compression ratio - whereby80% of the theoretical density was already indicated as sufficient andthese sintered bodies are isostatically hot-pressed at temperatures ofabout 200° C. above the melting point of the conductive component,copper. For this, the sintered bodies must be encapsulated under avacuum to prevent air or gas from being occluded in the material poresand to prevent the chromium from being oxidized by the residual oxygencomponent in the pressure gas. Moreover, the encapsulation can preventthe internal armature of the pressing device from being contaminated byan overflowing liquid phase.

As is true of single-axial hot-pressing, negligibly low residualporosities are also produced with isostatic hot-pressing (HIP). However,the indicated HIP process is not economical for the industrialproduction of high numbers of pieces. Encapsulating the sintered bodiesunder a vacuum entails a cost-intensive production step; hot-pressing inthe liquid phase, as indicated by the DE-A-37 29 033 as particularlyadvantageous, requires costly machining work to manufacture the contactfacings.

Furthermore, the DE-A-35 43 586 mentions a hot-isostatic pressing ofencapsulated blanks to produce contact materials on the basis of copperand chromium. However, this publication stresses that this processshould not be regarded as a recommendable production process, but rathermerely as a process for manufacturing reference specimen with fewresidual pores, that is only in special cases which justify such anexpenditure.

It is also known from the general art of machine construction tomanufacture parts with complicated contours using powder metallurgicalmeans in process steps as disclosed in JP-A-58-37 102 (Patent Abstractsof Japan Vol 7, No. 120, Mar. 25, 1983). In this process powder isinitially introduced into a flexible form under the pressure influenceof a liquid. The molded components are subsequently sintered in areductive atmosphere to a density of ≧93.5% and the sintered body issubjected to a hot-hydrostatic pressing operation, through which itobtains a density of ≧99%. This process has not previously been used toproduce contact materials with copper and chromium since it was believedthat it would entail a decisive reduction in quality. This was believedbecause of the high reactivity of chromium with oxygen since chromiumoxides decisively worsen switching properties in a vacuum switch.

The present invention seeks to solve the problems of prior artprocessing methods. The present invention provides a process forproducing CuCr contact pieces for vacuum switches of CuCr material,which provides an excellent material quality with a residual-porecomponent of <1% and which, at the same time, is inexpensive andeconomical when applied to the manufacturing of contact pieces from thematerial. In particular, this process should enable one to apply amolded-component technique with contours close to the final form and todispense with costly measures, such as vacuum encapsulation.

According to the present invention a powder blank is compacted in twosteps, whereby the first step is a sintering process with a compactionuntil a closed porosity of the sintered body, and the second step is ahot-isostatic pressing operation (HIP), in which the workpieces arebrought unenclosed to a final density of at least 99% space filling.

A closed porosity is achieved with the CuCr material produced accordingto the invention with sufficient reliability as of about 95% spacefilling. For an HIP operation, the closed porosity is necessary forworkpieces which are not encapsulated, to achieve the nearly completecompaction indicated according to the invention.

With the process according to the invention, a mixture of Cu powder andCr powder can advantageously be pressed into a blank whose form alreadyapproaches to the greatest possible degree the geometry of the desiredcontact piece or of the required contact facing. In accordance with theindicated two-step process, this blank is sintered under a vacuum and/orunder a reductive atmosphere in a solid Cu-phase and finallyisostatically hot-pressed in a solid Cu-phase.

Contrary to the previous concept, it is crucial for this processsequence that the hot-isostatic pressing make do without encapsulatingthe CuCr workpieces. Experiments demonstrated in particular that, evenwithout encapsulating the CuCr blanks during the process sequenceaccording to the invention, additional gases are not occluded nor doesthe chromium oxidize inside the material. It was discovered that, due tothe O₂ residual concentration in the pressure gas, the chromium onlyoxidizes on the surfaces of the workpieces. However, these outersurfaces are removed anyway when the contact pieces are finished. Bysintering the blank under a vacuum or a reductive atmosphere, areduction in the gas content is already achieved before the hot-pressingoperation. However, this is not the case when encapsulated powdermixtures are hot-isostatically pressed or when blanks arecold-isostatically pressed and subsequently encapsulated.

Contact pieces produced with the process according to the invention havea high material quality due to the homogenous distribution of thecomponents, their high compression and extremely low porosities. Fromthis and from the compression and hardening of the material achieved bymeans of the hot-isostatic compression process, result the desiredexcellent contact properties, such as high breaking capacity, dielectricstrength and resistance to erosion.

The cost-favorability of the process according to the present inventionhas to do, in particular, with the omission of the vacuum capsule andfurthermore with the fact that by sintering and hot-pressing in a solidphase, the contour of the blank is able to be selected to be very closeto the desired final form, so that only a minimal surface reworking isneeded. It is thus equally ensured that the amount of utilized materialis minimized.

The process according to the present invention can be advantageouslyrealized by applying a combined sintering-HIP process, in which powdercompacts of copper and chromium are initially sintered to low-porosity,in a vacuum or under H₂, and are subsequently isostatically hot-pressedin the same operation.

Composite parts can also be advantageously manufactured with the processaccording to the present invention: for example, contact facings of CuCrcan be produced at the same time with the contact carriers of Cu, astwo-layer or dual-area parts in one process sequence. One canconsequently dispense with the bonding production step--usually thehard-soldering in the vacuum. This is an important advantage,particularly for the application of bases made of solid Cu, since thesebases cannot be adequately bonded with the powder-metal compact by asintering process alone.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a first contact piece in cross-section;

FIG. 2 illustrates a second contact piece in a perspective view;

FIG. 3 illustrates a contact piece with a contact-piece base in aperspective view; and

FIG. 4 and FIG. 5 illustrate structural patterns of the material, beforeand after the hot-isostatic pressing.

DETAILED DESCRIPTION EXAMPLE 1

Electrolytically produced Cr powder with a particle-size distribution of<63 μm is dry mixed with Cu powder of a particle-size distribution of<40 μm in the proportion 40:60 and pressed into rings of the dimensionφ_(a) 60/φ_(i) 35×6 mm, single-axially with an applied pressure of 800MPa. The compacts are sintered at 1030° C. for 1 h under hydrogen with asaturation temperature of -70° C. and subsequently for 7 h under a highvacuum with a pressure p<10⁻⁴ mbar.The sintered bodies are subsequentlyhot-isostatically pressed at 950° C. for 3 h with 1200 bar under argon.The desired contact rings can be obtained simply by finish-turning theblanks.

EXAMPLE 2

A powder mixture of 25 m % aluminothermically produced Cr powder withparticle-size distributions of between 45 and 125 μm and 75 m % Cupowder with a particle-size distribution of <40 μm is pressed with apressure of 600 MPa on to a base of Cu powder with a particle-sizedistribution of <63 μm. A two-layer compact 1 is formed according toFIG. 1 with a disk-shaped Cu layer 2 and a truncated-cone shaped CuCroverlay 3 with a contact surface 4. The compact 1 is sintered at 1050°C. for 6 h under a high vacuum at a pressure of <10⁻⁴ mbar andsubsequently hot-isostatically pressed at 980° C. and 1000bar argon forabout 3 h.

In a variant of this example, besides copper and chromium, thepowder-metalcompact can also contain high-melting components such asiron (Fe), titanium (Ti), zirconium (Zr), niobium (Nb), tantalum (Ta),molybdenum (Mo), or also alloys of these components. In addition,readily evaporativeadditives, such as selenium (Se), tellurium (Te),bismuth (Bi), antimony (Sb) or their compounds, can also be contained.

EXAMPLE 3

A powder mixture corresponding to Example 1 is pressed with a pressureof 600 MPa into disks and sintered under a high vacuum with a pressureof <10⁻⁴ mbar at approximately 1060° C. already in the HIP deviceforabout 4 h. Immediately after that, it is hot-isostatically pressedwith500 bar argon at 1030° C. for about 2 h.

EXAMPLE 4

A powder mixture of 60 m % Cu powder with particles sizes of <63 μm and40 m % Cr powder with particle sizes of <150 μm is pressed with 750MPainto truncated-cone shaped, contact disks 5, according to FIG. 2,with contact surfaces 6. At the same time, slot contours 7 are impressedduringthe pressing operation, perpendicularly to the pressing direction.The sintering and HIP processes are conducted as in Example 2.

As a variant, one can also use a layered structure with a CuCr powdermixture for the contact facing and a Cu powder layer to produce a basewith excellent soldering capability, as described under Example 2.

EXAMPLE 5

A powder mixture corresponding to Example 4 is pressed with 800 MPa intoa flat, cylindrical contact facing 8 according to FIG. 3, and placedbefore the sintering process on a disk-shaped base 9 consisting oflow-oxygen or oxygen-free (OFHC) copper. During the sintering process,which is carried out at 1060° C. for approx. 5 h, the compact 8 and theCu-disk 9 bond together through sintering bridges. During a subsequentisostatic, hot-pressing step corresponding to Example 1, the compact 8and the copperdisk 9 bond so that the result is adequate compactness atthe boundary layer. When the contact piece is used in normal operation,the copper baseis able to be formed as a contact carrier or alsodirectly as a current-supplying bolt 10.

In the described process for manufacturing contact pieces, thecombination of the sintering and hot-pressing step is decisive forguaranteeing a highmaterial quality. As a result of the closed porosityafter the sintering process, there is no noticeable intercalation of airin the material during the HIP operation. This can be confirmed bymeasurements from the following table:

    ______________________________________                                                         O.sub.2 /ppm                                                                         N.sub.2 /ppm                                          ______________________________________                                        CuCr40, sintered state                                                                           534      14                                                CuCr40, hot-pressed state                                                                        532      19                                                ______________________________________                                    

Thus, the oxygen and nitrogen contents lie in the same order ofmagnitude before and after the hot-isostatic pressing of the unenclosedworkpieces.

It becomes clear from the corresponding structural patterns thatchromium particles 12 are embedded at any one time in a copper matrix11, whereby in the sintered state in FIG. 4, blank spaces 13 still occurnow and again. However, they are sealed to the outside due to the closedporosities. On the other hand, FIG. 5 confirms that by means of furtherisostatic compression, the blank spaces 13 in the CuCr material arecompletely eliminated. Consequently, a nearly compact material with aspace filling of more than 99% now exists, and this material wasmanufactured in a comparatively simple manner.

We claim:
 1. A process for producing a vacuum-switch contact piece withcopper and chromium, in which a powder blank is compacted, comprises thesteps of:compacting the powder blank in two stages,in the first stagesintering to compact until a closed porosity of the sintered body isachieved; and in a second stage performing a hot-isostatic pressingoperation for sintering the solid state of the copper-chromium compact,wherein the sintering process takes place at a temperature in the rangeof between 1000° C. and 1070° C. and the hot-isostatic pressingoperation takes place under inert gas below the melting temperature ofcopper (1083° C.), and that the sintered body is brought unenclosed to afinal density of at least 99% space filling.
 2. The process according toclaim 1, wherein the sintering process and the HIP process are carriedout immediately one after the other, without any intermediate cooling,in a device for hot-isostatic pressing.
 3. The process according toclaim 1, wherein the sintering process is carried out in a high vacuumin the pressure range of ≦10⁻⁴ mbar.
 4. The process according to claim2, wherein the sintering process is carried out in a high vacuum in thepressure range of ≦10⁻⁴ mbar.
 5. The process according to claim 1,wherein besides in a vacuum, the sintering is also carried outtemporarily in pure hydrogen with a saturation temperature of <-60° C.6. The process according to claim 2, wherein besides in a vacuum, thesintering is also carried out temporarily in pure hydrogen with asaturation temperature of <-60° C.
 7. The process according to claim 1,wherein during the hot-isostatic pressing (HIP), the inert gas is argonor helium.
 8. The process according to claim 2, wherein during thehot-isostatic pressing (HIP), the inert gas is argon or helium.
 9. Theprocess according to claim 8, wherein the hot-isostatic pressing (HIP)is carried out at pressures of between 200 bar and 2000 bar.
 10. Theprocess according to claim 7, wherein the hot-isostatic pressing (HIP)is carried out at pressures of between 200 bar and 2000 bar.
 11. Theprocess according to claim 1, wherein a powder compact consisting of ahomogeneous mixture of copper and chromium with 25 to 40 m % Cr is used.12. The process according to claim 1, wherein a powder compact is used,which in certain regions consists of a homogeneous mixture of copper andchromium with 25 to 40 m % Cr.
 13. The process according to claim 12,wherein in addition to regions with CuCr mixtures, the powder compactalso contains regions of pure Cu powder.
 14. The process according toclaim 1, wherein a powder compact is used, which in certain regionscontains a powder mixture of copper (Cu), chromium (Cr), and one or moreadditional high-melting components such as iron (Fe), titanium (Ti),zirconium (Zr), niobium (Nb), tantalum (Ta), molybdenum (Mo), or alloysfo these components.
 15. The process according to claim 12, wherein apowder compact is used, which in certain regions contains a powdermixture of copper (Cu), chromium (Cr), and one or more additionalhigh-melting components such as iron (Fe), titanium (Ti), zirconium(Zr), niobium (Nb), tantalum (Ta), molybdenum (Mo), or alloys fo thesecomponents.
 16. The process according t claim 8, wherein a powdercompact is used, which in certain regions contains a powder mixture ofcopper, chromium, and other readily evaporative additives, such asselenium (Se), tellurium (Te), bismuth (Bi), antimony (Sb) or theircompounds.
 17. The process according to claim 12, wherein a powdercompact is used, which in certain regions contains a powder mixture ofcopper, chromium, and other readily evaporative additives, such asselenium (Se), tellurium (Te), bismuth (Bi), antimony (Sb) or theircompounds.
 18. The process according to claim 1, wherein a powdercompact is manufactured with a radially symmetrical geometry, forexample, a ring, a disk, or a truncated cone, close to the finalgeometry of the finished contact piece.
 19. The process according toclaim 1, wherein a powder compact is manufactured with cutouts or slotsparallel to the pressing direction.
 20. The process according to claim1, wherein the powder compact is sintered in the first stage on to asolid base and that, in the second stage, at the same time as thecompaction toward end porosity, an intimate bonding between the sinteredbody and the solid base is produced.
 21. The process according to claim20, wherein a contact stud of low-oxygen or oxygen-free (OFHC) copper isused as a solid base.